In conventional electric discharge machining, a gap is formed between an electrode and a conductive workpiece to be machined (hereafter, referred to as "workpiece"). The gap is filled with an insulating medium and intermittent electric discharges are passed between the electrode and the workpiece. The insulating medium is an aqueous or non-aqueous electric discharge machining fluid. It is desirable that the machining fluid also help promote the safety of the machining process.
In industrial practice, electric discharge machining often takes place through continuous operation in a completely automated plant. Furthermore, in the practice of electric discharge machining, the surface temperature of the workpiece may rise excessively (e.g., to thousands of degrees). Accordingly, it is desirable to use an aqueous electric discharge machining fluid which will not lead to or increase the probability of fire.
There are, however, several disadvantages associated with the use of a non-flammable aqueous electric discharge machining fluid and, therefore, its use is limited in industrial practice. The disadvantages include as follows:
(a) As machining occurs, the electrode is worn away as well as the workpiece. It is indispensable, for precision machining, however, that the electrode wear rate be kept at or below 1%. During normal machining conditions using aqueous electric discharge machining fluid, the electrode wear rate is remarkably high and thus it is difficult to conduct precision machining.
(b) Uneven wear of the electrode also occurs during machining due, for example, to the electrolytic action which occurs when a plus potential is applied to a tool electrode while a minus potential is applied to the workpiece to be machined. The machining accuracy worsens as a result of uneven electrode wear.
(c) Machined iron particles mixed in the machining fluid as well as newly formed products can cause undesirable phenomena such as partial electrode wear, abnormal electrode wear and reduced resistivity. Consequently, abnormal discharges may occur and damage a workpiece. There may be no means, however, to prevent the formation of products which cause the abnormal electric discharges. As a result, machining fluid is circulated through a filtering apparatus and ion-exchange resins to remove the machined iron particles and the newly formed products as part of the recycling process. However, the ion-exchange resins conventionally reach their working capacity limit within a short period of time. The machining fluid must, therefore, be replaced frequently.
(d) Since the main ingredient of the aqueous electric discharge machining fluid is water, an unavoidable, partial electrolysis of water is involved during the electric discharge machining. During the electrolysis of the water, oxygen is produced forming an undesirable machining product.
The addition of additives and the like to the aqueous electric discharge machining fluid to improve the machining removal rate has not previously been effective since the ion-exchange resins have heretofore captured such additives. Therefore, although additives to improve the machining removal rate have previously been disclosed in Japanese Patent Publication No. 4253/1984 and Japanese Patent Application Laid-Open Nos. 94223/1987, 71120/1985, 4623/1986, 236623/1987, 236625/1987 and 2618/1988, these additives are not used in industrial practice because they are captured by ion-exchange resins.
Furthermore, it is generally recognized that the addition of material to the machining fluid for preventing the formation of products resulting from electric discharge machining is technically insignificant. Thus, the addition of such matter is not being carried out in industrial practice.
Thus, it is a problem in the prior art to provide a machining fluid which solves the above disadvantages, while maintaining electrode wear rate below 1% and greatly improving the machining removal rate.